Ablation behavior of high-entropy transition and rare-earth metal diborides (Hf_0.2Zr_0.2Ta_0.2Nb_0.2RE_0.2)B₂ (RE = Sc, Lu, Tm)

Transition metal diborides (TMB₂) are promising ultra-high temperature ceramics, but their application is limited by degradation of the oxide layer above 1600 °C due to phase transformation, cracking, and volatilization. To address this issue, this study investigates the ablation behavior of HE TM_0.8RE_0.2B₂ (RE = Sc, Lu, Tm) through theoretical calculations and experimental verification. The mass ablation rates are −0.17, −0.20 and −0.22 mg·cm⁻²·s⁻¹, and the linear ablation rates are −13.18, −12.45 and −12.70 μm·s⁻¹ for RE = Sc, Lu and Tm, respectively. Microstructural analysis reveals a biphasic oxide scale consisting of solid‑phase multi‑component oxides and a molten (Nb, Ta)₂O₅‑based phase. The solid‑phase oxides form a dense high‑entropy oxide (FMO_2-δ) through mutual solid solution, while the molten phase fills pores and cracks generated by B₂O₃ volatilization, providing an in‑situ self‑healing capability. The phase structure of FMO_2-δ depends on the RE element: Lu and Tm fully stabilize the tetragonal ZrO₂‑type structure, whereas Sc only partially stabilizes it, leading to a mixed monoclinic/tetragonal structure. First‑principles calculations and thermodynamic analysis confirm that all constituent oxides have a lower solid‑solution barrier in the ZrO₂‑type lattice than in other structures, driving the formation of a continuous and dense oxide layer. This unique “dense skeleton/liquid‑phase‑filled” architecture endows the HE TMREB₂ ceramics with exceptional ablation resistance at ultra‑high temperatures.